1. Field of the Invention
The present invention relates generally to the field of measuring magnetic fields. More specifically, the present invention relates to measuring low-level magnetic fields with gradiometers and magnetometers using superconducting quantum interference devices (SQUIDs) in unshielded environments.
2. Discussion of the Prior Art
A superconducting quantum interference device (SQUID) magnetic sensor is at the heart of many sensitive magnetometers aimed at measuring magnetic fields below about 10−12 Tesla (T). This level is within the range, for example, of magnetic fields produced by living organisms (often referred to as biomagnetic fields).
SQUIDs are sensitive to magnetic flux ΦB. Magnetic flux ΦB may be defined as the projection of the average magnetic field threading a given area along a vector z positioned normally to the area, or mathematically:ΦB=Bz*A  (1)
A low-Tesla direct current (DC) SQUID typically consists of two nominally identical Josephson junctions serially connected in a superconducting, electrically continuous loop, typically on the order of 10−4 to 10−2 millimeters (mm) in diameter. SQUID-based magnetometers and gradiometers are among the most sensitive magnetic field detectors in current use.
SQUIDs are typically produced on chips, using Niobium-Aluminum-Aluminum Oxide-Niobium (Nb—Al—AlOx—Nb) junction technology, with associated junctions and the SQUID loop formed from thin films. The micron-scale features of the device may be formed using photolithographic techniques. The SQUID chip is typically enclosed in a superconducting shield for screening the device from ambient magnetic flux. The magnetic flux to be measured is typically intercepted by considerably larger diameter loops or coils (for example, 10-20 mm) inductively coupled to the SQUID via an input coil. These larger coils are usually made of thin insulated superconducting wire (for example, Nb) wound over an insulating cylindrical support. A device having a single coil or loop may be referred to as a magnetometer, a device having combinations of more than one coil or loop may be referred to as a gradiometer. Such a device is schematically illustrated, for example, in FIG. 4.
Unfortunately, this unprecedented sensitivity of SQUID-based gradiometers comes at a price, as sensitivity can cause the device to become overwhelmed by ambient noise and to stop working when exposed to radio frequency interference (RFI). As a result, these devices can often only be operated in heavily shielded enclosures, which may be impractical for all real life applications. Accordingly, it would be quite beneficial if SQUID-based devices could be operated without such shielding. If we can do this then we have a practical sensitive magnetic field measuring instrument. The present application discloses a method and apparatus for achieving this goal.
In the presence of RFI, a SQUID may lose sensitivity or even cease to function. Sources of strong RFI include, for example, ultrasound machines in hospitals, AM and FM radio signals, TV and cellular communications transmissions. At frequencies lower than the RF band, SQUID electronics can be used to measure the noise, and known software techniques can be used to eliminate this noise (e.g., Bick et al., “SQUID Gradiometry for Magnetocardiography Using Different Noise Cancellation Techniques”).
A simple but often impractical solution to RFI elimination in prior art systems has been to surround the system with a few layers of fine copper mesh and isolate the area of operation. The copper mesh reduces RFI considerably, but doesn't eliminate it, as it is difficult to cut down transmission through connecting cables. While the SQUID itself may be suitably shielded inside a small Nb tube, an unshielded gradiometer that picks up the measurement signal and feeds it to the SQUID will nonetheless couple RFI into the SQUID, as the gradiometer itself cannot be shielded (otherwise it cannot pick up the signal to be measured). Thus, there is a need for a technique that allows the gradiometer to couple the signal of interest into the SQUID, without additionally coupling the RFI into the SQUID.
The following references provide for a general description of prior art SQUID systems, but they fail to provide effective means for shielding the SQUID from radio frequency interference.
The Japanese patent to Fujimaki (JP 4212079) provides for a SQUID magnetic field sensor, wherein damping resistors R1 and R2 are used to eliminate only the magnetic part of the RFI.
The non-patent literature to Ishikawa et al. entitled, “Effect of RF Interference on Characteristics of a DC SQUID System”, and Koch et al (Appl. Phys. Lett., vol 65, pp. 100-102) entitled, “Effects of radio frequency radiation on the dc SQUID,” provide background information related to RFI interference in SQUID systems.
The non-patent literature to Bick et al. (“SQUID Gradiometry for Magnetocardiography Using Different Noise Cancellation Techniques”), and Tarasov et al. (“Optimization of Input Impedance and Mechanism of Noise Suppression in a DC SQUID RF Amplifier”) illustrate, in general, the use of noise cancellation techniques with a SQUID device.
The U.S. patent to Simmonds (U.S. Pat. No. 5,319,307) covers improving SQUID performance. References to a superconducting shielding layer are directed to shielding the SQUID chip from RFI, and it should be noted that in general all SQUIDs, even those used in shielded enclosures, are kept inside a superconducting Nb tube with gradiometers connected from the outside through small holes in the Nb tube.
The U.S. patent to Colclough (U.S. Pat. No. 5,532,592) covers electronics (flux-locked loops) in multichannel systems. It should be noted that the reference to a brass enclosure is a routine procedure in electronics to shield against RFI; but this procedure is inadequate against RFI transmitted through a wire that goes through enclosures.
The U.S. patent to Seppä (U.S. Pat. No. 6,066,948) discloses damping individual junctions of a SQUID. It should be noted that this is a common procedure and more information regarding this procedure can be found in the book by Weinstock entitled, “Applications of Superconductivity” (Kluwer, Netherlands, 2000). It should further be noted that this procedure allows for the damping of internal oscillations of junctions that affect operation of the SQUIDs and does not reduce RFI coupled to the SQUID itself.
The U.S. patent to Steinbach et al. (U.S. Pat. No. 6,169,397) describes a method for damping internal resonances of the SQUID. The damping helps shield the SQUID from magnetic part of the RFI and is similar to the Japanese patent by Goto (JP 4160380) that provides for a general background in noise suppression techniques as implemented in prior art SQUID systems.
Furthermore, the Japanese patent to Kawai (JP 7198815) appears to teach along the same lines as that of the Steinbach et al. patent.